Apparatus for heat-treating metal articles



Patented June 24, 1936 UNITED STATES THDMAS R. HEYWARD, JR., OFPITTSBURGH, PENNSYLVANIA APPARATUS FOR HEAT-TREATING METAL ARTICLES NoDrawing.

This invention relates to the art of heat treating metals. It isconcerned not only with new and improved apparatus for heat treatingmetals, particularly steel in the '5 form of elongated relatively thinarticles such as sheets or strips, but it is also concerned with a newand improved step or steps in the heat treatment of metals and in theoperation of furnaces for heat treating metals in the above describedform.

Heretofore thin elongated metal articles, particularly steel. sheets andstrips, have been heat treated by being passed on waster sheets,composed of iron or steel and, for example, of 18 gauge thickness,through heat treating furnaces provided with suitable conveying meanswhich engaged and carried the waster sheets.

The'waster sheets have been employed to space the articles being treatedfrom the conveying mechanism in the furnace primarily for the purpose ofpreventing pittings or surface defects on the articles which oftenresults from the presence of lumps in the form of complex oxides ofconsiderable hardness adhering to the conveying mechanism. These wastersheets, which are usually composed of steel, oxidize and scale veryrapidly, due to the high heat maintained in the furnace so that after avery fewpasses through the furnace the surfaces of the waster sheetsbecome more or less completely covered with scale which flakes ofi asthe steel wasters are repeatedly used. Such scale particularly on thelower surface of the sheet is dislodged from time to time by strikingthe conveyor mechanism and falls below the conveyor mechanism where itaccumulates on the hearth of the furnace. This results in that theamount of heat reflected form the hearth is decreased and in time thehearth is so filled with scale that the furnace must be shut down,cooled ofi and the scale removed, a procedure that requires several daystime.

The scale on the upper surface of the steel waster sheet, which comes incontact with the metal articles being heat treated, is harder at thehighest temperatures than the sheets being treated and tends to scratchApplication filed July 19, 1929. Serial No. 379,584.

or form pits in the metal articles. The scaled waster sheets also arerelatively poor conductors of heat as compared with the unsealed wastersheet. Consequently the heat from the waster sheet is-not transmitteduniformly throughout the entire surface of the waster sheet to thearticles carried thereon with the result that the articles which rest onsuch scaled waster sheets are non-uniformly heated and consequently thephysical properties of the resultant articles are not uniformthroughout. This nonuniformity of heating destroys theprime reason ofsuch a furnace as it is highly desirable that the articles should beuniformly heat-treated throughout and that the physical propertiesshould be substantially the same throughout the articles.

Moreover, the iron or steel waster sheets used. heretofore have anexceptionally short life, due to the scaling tendency, and such wastersheets which have been passed through a continuous normalizing furnacefor perhaps less than two hundred times are entirely useless for furtheruse. In addition to this rapid deterioration of the iron or steel wastersheets, these sheets are dificult to handle after they have passedthrough a furnace a small number of times because of the formation ofloose and splintery scale on the edges, together with the surface scalethereof which readily cut the hands of workmen who handle the wastesheets at the exit and entering ends of the furnace and likewise due totheir roughness of scale and splinters often scratch the ar ticles asthey are removed from the waster at the exit end of the furnace.

The usual prior practice with steel waster sheets has been to use sheetsof about 18 gage, (.050) to 20 gage (.037). They can be used longer thanthinner sheets before the scaling has reduced them to such a thinnessthat the incrustations on the conveyor will penetrate through them andpit or mar the article thereon. An important disadvantage, however, isthat the steel waster sheets will thus vary in thinness and amount ofscale. For example, after some use certain, sheets will be discarded asno longer useful and replaced by new sheets. Then the old sheets may bereduced to 20 gage (.037) or less while the new sheets are thicker andcarry less scale. As time goes on the sheets for a given furnace may beall of different thicknesses and of different degrees of scaling. Theresult is that the articles being treated are not uniformly heated andthe product will vary in physical properties.

By my invention I start with thinner carrier sheets, for example 24 gage(.025) which, due to-their properties, resist penetration by theincrustations as well or better than the thicker steel waster sheets,andrepeatedly use my carrier sheets many times as long as the steelwaster sheets can be used so that replacements are decreased and theresultant product is of a much more uniform character.

Furthermore, a scaled steel waster sheet does not dissipate heat asrapidly as my nonscaling carrier sheet. This has a marked effect ongrain growthin the article being heat treated, particularly innormalized sheets or strip metal, and results in larger grains in theparts of articles contacting with the scaled steel waster than in theupper or more quickly cooled metal, the variance in grain growthdepending on the thickness and amount of scale present on the-steelwaster sheet. With my alloy carrier sheets the heat is dissipatedrapidly therefrom and:

undesired grain growth is prevented in the metal of the articlescontacting therewith.

Wide differences in results between the steel waster sheets and my alloycarrier sheets is even more pronounced than above noted when two or moremetal article sheets are carried through the furnace in stacked relationin that the heat dissipation from the bottom article sheet is retardedby the article sheets thereabove and must, therefore be increasedthrough the alloy carrier or steel waster sheets in order to obtain thesame physical and microstructural pro erties' in the bottom sheet as areobtalne I in the article sheets thereabove. My alloy carrier sheets aremuch better adapted to such heat dissipation than the steel wastersheets whether ornot the latter are scaled.

By means of the present invention I am able to overcome thedisadvantages present in thewaster sheets of the prior art, to obtain acarrier sheet which will be much thinner than the steel waster sheets,and have many times the length of life of the prior art waster sheets,and which will not only be easier to handle, due to the lighter weightand absence of slivers andscale, but which,

will also be capable of transmitting heat more quickly than the steelwaster. sheets and also substantially uniformly throughout t ent re srface t t e t c es in c n a t with steel carrier sheets which scalebadly and with sulphur containing fuel the, conveyors rapidlydeteriorate. With my pres- 1 ent non-scaling carrier sheets, however,the scale is practically eliminated and the life of the carrierscorrespondingly rolonged, as the action of the sulphur gas a one is lessdetrimental than sulphur gas in the presence of iron oxide.

In practicing my invention I form a relatively thin carrier sheet of,for example, 24 gage metal and of the desired size and configuration ofa metal or metal composition, which is highly resistant to oxidation athigh tern eratures as compared with iron or steel. guch a compositionwhich has been found in use to be satisfactory for long continued use,consists predominantly 0 iron and contains from fifteen percent to aboutthirty percent of chromium. Other suitable compositions consist of ironwith about fifteen percent to thirty percent of chromium and from aboutfive to about fifteen percent of nickel. Part of the chromium may bereplaced by nickel. One preferred composition consists of iron withabout twentyeight per cent of chromium due to its greater resistance tooxidation at high operating temperatures and also to greater resistanceto the varying fuel compositions. Another satisfactory compositioncontains about eighteen percent chromium and eight percent nickel.

My improved carrier sheets are em loyed in the method of operating ameta heat treating furnace or in the method of continuously heattreating articles as for illustrative purposes, a continuous normalizingfurnace with a process as follows.

I When a. normalizing furnace of thecontinuous type that is a type inwhich the articles movefrom the entrance to the exit of the furnace isbeing used for continuously normalizing sheet metal, alloy carriersheets of my invention are fplaced on the conveyor mechanism of theurnace at the chargmg end and the sheet articles to sheets thus "spacethe articles-from the conveyor mechanism which would mar the surface ofthe article due to the presence of oxide lumps previously described. Thecon- ,vieyor mechani sm then moves the carrier sheets and articles th onthr ugh he nace, and both the articles and carrier sheets are heated tothe desired temperature and then cooled. When the carrier sheets andarticles emerge from the exit end of the furnace the articles areremoved from the carrier sheets, which are then ready for furthertreatment or use. The carrier sheets are then returned to the chargingend of the furnace for a repeat of this process.

During the passage of the carrier sheets and articles thereon throughthe furnace, the carrier sheets are heated to approximately thetemperature of the articles thereon and the heat thus absorbed by thecarrier sheets is transmitted substantially uniformly from the entireupper surface of the carrier sheets to all surfaces of the articlescontacting therewith. This results in the latter being uniformly heatedby the carrier sheets and in the articles being substantially anduniformly heated treated throughout. As the articles cool in the coolingzone of the furnace they transmit heat to the carrier sheets forsubstantially the entire upper surface of the latter and such heat isdissipated by the carrier sheets more or less uniformly from its lowersurface so that substantially the entire surface of the articles incontact with the carrier sheets is uniformly cooled and certain partsare not cooled more slowly nor more rapidly than other parts as oftenoccurs with badly scaled steel waster sheets.

Moreover the. chrome-iron carrier sheet absorbs and dissipates its heatmore rapidly due to its thinness, and its freedom from scale than asteel waster which is a marked advantage. My thinner non-scaling alloycarrier sheet also shapes itself to the articles thereon and brings moreof its non-scaled surface into contact with the articles more quicklythan does the thicker and scaled steel waster sheets and consequentlyheats the articles thereon more completely, quickly and uniformly thandoes the steel sheets. Having thus described my invention so that thoseskilled in the art may be enabled to practice the same, what I desire tosecure b Letters Patent is defined in what is c aimed. It beingunderstood that various changes in the foregoing detailed descriptionmay be made by those skilled in the art without departing from thespirit and scope of my invention.

What is claimed is 1. A carrier sheet for use in a heat treating furnacecomprising iron containing between about fifteen percent and aboutthirty 2. In combination with a continuous heat treating furnace andconveyor means in the furnace for moving through the furnace metal to beheat treated, a carrier sheet engageable with and carried by theconveyor means and adapted to carry thereon and through the furnacemetal articles to be treated, the carrier sheet being composed of analloy having high resistance to oxidation and scaling as compared toiron or steel at the temperatures to which it is subjected in thefurnace, and having throughout substantially its full serviceable lifethe propertyof transmitting heat to articles thereon substantiallyuniformly to and from all surfaces of the articles being treated whichmake contact with the alloy carrier sheet.

In testimony whereof I hereunto afiix my signature this 13th day ofJuly, 1929.

THOMAS R. HEYWARD, JR.

